This invention relates to a mat for supporting a person in an upright position comprising a number of adjacent interconnected liquid-filled compartments said compartments being dome shaped on at least one side of the mat and the dome-shaped compartment walls being mainly made from an elastomeric material.
A supporting device of the above type is disclosed in EP patent publication No. 0 170 947 A1.
Mats of the type described above have become popular for use in places of work where one stands up during work, e.g. persons tending machines and shop personnel, because said mats have proved to prevent tiredness in and swelling of the legs.
Experience has shown, however, that such mats slacken after a while with the result that one can "step through" the mats and that they are no longer comfortable to stand on.
In an attempt to eliminate this drawback and to achieve an increased degree of comfort for the users of said mats a closer analysis has been performed of the conditions determining whether the mats in question are comfortable to stand on or not.
During such analysis, it has been found that the reason why an erect person at work quickly experiences tiredness in his legs and suffers from swelling of his legs is that only relatively few muscle groups are activated and that, on the other hand, said muscle groups are strained for relatively long periods. This concurs with the seeming paradox that one feels more tired when standing completely still for a long time than when moving around, even though the latter presupposes higher energy consumption.
The actual reason why one by standing still rapidly feels tiredness in one's legs appears to be that the blood flow through the muscle tissue is reduced. Reversely, it is to be expected that if a person standing on a mat of the type in question is constantly forced to change position an activation of an increased number of muscle groups and thus an increased blood flow will be the result.
The result of changing positions will not only be important with respect to the leg muscles but also to foot joints, knee joints and hip joints which will also change position repeatedly.
Further investigations of the prior art mats have shown that the reason why they slacken so quickly is that the liquid pressure inside the mat decreases. This is more likely due to a change in the elastomeric material than to a loss of liquid by vaporization or leakage.
Attempts have been made to prevent the mats from slackening by increasing the pressure within the liquid-filled compartments to about 2.5 bar, which is substantially higher than the initial pressure in the prior art mats. Such a high pressure entails, however, that the mats feel too hard at the beginning which is unacceptable.
The pressures indicated herein are all pressures above atmospheric pressure.
Attempts have also been made to avoid a change in the elastomeric material by employing heat treated rubber plates in the production of said mats. This attempt has not met with the desired results because mats produced from such plates also slacken after a while.
Surprisingly it has been found, however, that if a pressure of 2.5 bar is established within the liquid compartments and the mat is subsequently subjected to heat treatment, a stable condition is achieved subsequent to an initial controlled change and the mat becomes resistant to long term changes. The heat treatment causes the pressure within the compartments to decrease to a suitable value sufficiently high to prevent "stepping through" but, on the other hand, not so high that the mat is felt hard to stand and walk on.
The change in the elastomeric material causing the prior art mats to slacken is presumably due to the fact that the tensile stress prevailing within the elasomeric material of the mat falls drastically in the course of time. The elastomer is subjected to a relaxation process. As to the prior art elastomers it has been found that the material relaxes with 10-15% per time decade, i.e. the relaxation--the fall in the tensile stress expressed in percentages--during the first 24 hours is the same as during the following 10 days and nights and during the next-following 100, etc.
The heat treatment of the liquid-filled mat, i.e. comprising an elastomeric material under heavy stress (as a consequence of a relatively high liquid pressure), apparently represents an accelerated relaxation and the relaxation cycle is hereby brought into a time decade wherein the relaxation per time unit is minor.
Apart from the fact that the internal pressure of the mat thus treated remains stable over the entire life time of the mat, it has acquired novel and useful properties especially as regards furthering the above mentioned muscle movements, which properties will be explained in the following.
Although the heat treated mats, as mentioned, are stable with respect to long term changes i.e. changes in the liquid pressure in the compartments, it has been found that a considerable relaxation of the material occurs in connection with the new loads being imposed on the material when a person steps onto the mat.
The load carrying capacity of the mat is a function of the liquid pressure within the compartments and the loaded area, and it is therefore to be expected a stable condition is achieved when a person stands with both legs on the mat. However, this is not the case because, as mentioned above, subsequent to the higher load a further relaxation is produced as evidenced by one's sinking down into the mat. Since the load ordinally is not the same from one leg to the other, one feel that one leg sinks lower than the other, and this generates a reflex induced correction in the attempt to change the load or to change one's position. More importantly, the same thing happens across the plane of the individual foot in case the heel or forefoot sinks.
These reflex conditioned changes result in the activation of other muscle groups and reduce, as explained above, the feeling of tiredness.
Such influences are normally of a relatively short duration and the subsequent relaxation is reversible. Despite the stabilisation of the material against long term relaxation, this allows for the material to undergo a desirable reversible relaxation of brief duration.
What is obtained is in other words controlled instability.
Furthermore, it has been found that a mat produced as explained above has a load carrying capacity which is 20-30% higher than that of a corresponding prior art mat. The term load carrying capacity means the force needed to achieve a given reduction of the height of the dome-shaped compartments over a support.